Process for drying electrical equipment disposed in a sealed casing



Sept. 27, 1955 RD DISPOSED IN A SEALED CASING Filed Oct. 26, 1951 J. G.FO PROCESS FOR DRYING ELECTRICAL EQUIPMENT To Vacuum Pump I4 v w r- I QA Heater Oil Punfler Trap I I g I8 I i 38 36 9' WITNESSES: INVENTORfi/flzr/ g James 6. Ford.

7} A W BY ATTOR EY 2,718,709 Patented Sept. 27, 1955 United StatesPatent ()fiice PROCESS FOR DRYING ELECTRICAL EQUIPMENT DISPOSED' IN ASEALED CASING James G. Ford, Sharon, Pa, assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa., 2 corpora! tion ofPennsylvania Application October 26, 1951, Serial No. 253,253

4 Claims. (Cl. 349) This invention relates to a process for rapidly andefiiciently dryingout electrical apparatus disposed in a sealed casing,prior to putting the apparatus in service.

At the present time, electrical apparatus disposed within a sealedcasing is subjected to an involved and prolonged treatment to removemoisture, gases and other undesirable impurities therefrom prior toputting the electrical apparatus in service. Cellulosic insulation, suchas paper, pressboard, cotton cloth or tape and the like, is employed asa component of the insulation of the electrical apparatus, and suchcellulosic insulation contains substantial quantities of moisture. Undernormal conditions, the cellulosic insulation will contain from 2% to1.0% by weight of moisture. Other components of the insulation alsocontain substantial amounts of moisture. If normal electrical currentwere to be applied to the electrical apparatus under these conditions ofhigh mois ture content, failure might occur because of the low.dielectric strength of the insulation due to such. large amounts ofmoisture being present.

It is an accepted procedure at the present time to thoroughly preheatelectrical apparatus in an air oven before placing it in the sealedcasing in which it will be disposed thereafter and to evacuate the hot,electrical apparatus when in the sealed casing in order to removemoisture and other undesirable volatile impurities. Such treatmentprovides for an adequate dielectric strength of the insulation towithstand operating voltages. Thereafter, the dried apparatus may beimpregnated with a dielectric liquid which serves to cool and toinsulate the electrical members.

The present drying: procedure, however, is time consuming, particularlywith the larger size of electrical equipment, such as powertransformers. From 3 to 5 days preheating in anair oven may be requiredtobring the temperature of the cores and coils of a large powertransformer to a temperature of 85 C. Some deterioration of thecellulosic insulation may take place since the air inthe oven is at atemperature of from 125 C. to 135- C. The preheated core and coil arethen removedfrom the heating oven, placed within a vacuum tank or, insome cases, in the transformer casing while still hot. Immediatelythereafter, the treating tank or transformer casing are subjected toevacuation in order to withdraw any moisture or othervolatiles from theinsulation of the transformer core and coils. The total time requiredfor drying out a 100,000 kva. transformer by this procedure is in theorder of from 7 to 8 days. No process known at the present time isavailable in which this drying out peri'odmay be substantially reduced.

When a transformer is being repaired afterhaving been in service, theinsulation is impregnated withoiland, upon being dried in the: airoven-infollowingthe drying out procedure, the oil will oxidize. Thisresults in an undesirable gummy sludge being present in the surfaces.Corrosive acids are also produced. Accordingly, such known proceduresfor drying out the. members. are. unsatisfactory.

The object of the present invention is to provide a process for-rapidlyand efliciently drying out electrical apparatus while it is enclosed ina sealed casing.

A further object of the invention is to provide for simultaneouslyheating and evacuating electrical apparatus disposed in a sealed casingby spraying hot dielectric liquids over the electrical apparatus whileevacuation is being carried out.

Other objects and advantages of the invention will in partbe obvious andwill in part appear hereinafter. For a better understanding of thenature and objects of the invention, reference should be had to thefollowing detailed description and drawing, in which the single figureis a view in elevation, partly broken, of a transformer being treated inaccordance with the invention.

I have discovered a process by means of which electrical equipmentdisposed in a sealed casing, particularly large electrical equipment,may be dried outin a fraction of the time required heretofore and withno damage or deterioration occurring. Briefly, the electrical elementssuch as coils or windings and magnetic cores, are disposed within thesealed casing in which they are to function thereafter, then hot oil orother dielectric liquid is, introduced into the casing and sprayed,flowed or otherwise distributed over the electrical elements proper inorder to impartheat to them. The oil running off the apparatus collectsat the bottom of the casing and is withdrawn, and the withdrawn oil ispassed through a purifying means and a heater inorder both to removeimpurities thatmay have accumulated therein and to reheat the oil, thenthe oil: is returned to be resprayed or otherwise applied totheapparatus. The casing is evacuated during this procedure.

Spraying, flowing or otherwise distributing hot oil or other liquiddielectric, has been found to heat. electrical members promptly withoutcausing any deterioration. of the insulation thereof. It is preferred touse petroleum oils and synthetic dielectric liquids, such as chlorinateddiphcnyl, for the spraying procedure to provide for heating of theelectrical, apparatus in the process. The oil or other dielectric liquidshould have a low vapor pressure at the temperatures to which it isheated while under a low vacuum of the order of 1 inch of mercuryabsolute pressure or less. Inasmuch as oil or other dielectric liquidwill be used to fill, the sealed casing after drying out has beenefiected, it usually is desirable to employ the same dielectric liquidfor the drying out of the transformer as will be employed thereafter inthe casing as the permanent liquid dielectric.

Referring to the figure of the drawing, there is illustrated atransformer 10 comprising a tank 12 and a cover 14 cooperating toproduce a sealed casing. Disposed Within the tank 12. is an electricalcoil 16 and a core 18 having. insulation which contains substantialamounts of moisture, oxygen and other gases and volatile impuritieswhich it is desired to remove before the transformer is put intoservice. Accordingly, there is introduced through the cover 14 a conduit20 carrying a hot dielectric liquid, such as oil. The conduit 20 isconnected to a series of nozzles or sprays 22 which distribute streams24 of the hot oil over the coil 16 and core 18. As the streams of hotoil flow over the surfaces of the coil and core, they impart heatthereto rapidly. The oil runs off to the bottom of the tank 12 andcollects as a small pool at a considerably lower temperature than insprays 24'. The oil collected in the bottom of the tank 12 is withdrawnby means ofa conduit 28 connected to a pump 30' which forces the oilinto a purifying device 32. Within the purifying device 32 is present anadsorptive material, such for example as fullers earth, charcoal,activated alumina or silica gel in the form of bricks, pellets orfragments. Any water or other impurities acquired by the oil in itspassage over the transformer coil and core is removed therefrom withinthe purifier 32. The purified oil then enters the conduit 20 whichpasses through a heater 26 illustrated as an electrical heating element.It should be also understood that the position of the heater 26 andpurifier 32 may be transposed, or they may be associated in one unit.Then the hot purified oil is returned to the spray heads 22 forrespraying whereby the coil 16 and core 18 are further heated to a stillhigher temperature. In a matter of a few hours large sized coils andcores may be heated to a suitable temperature under these conditions.

It is ordinarily sufficient to heat the coil and core to a temperatureof from 80 C. to 100 C. However, in some cases a temperature as low as50 C. may be suflicient. In other cases, where the apparatus is preparedfrom more heat resistance insulation than paper, for instance glasscloth, the temperature to be applied may substantially exceed 100 C.Therefore, the oil or other dielectric liquid used in heating theapparatus is heated to at least 50 C., and preferably to from 80 C. to110 C. or slightly higher.

Connected to the tank 12 is a vacuum pipe 36 leading to an oil trap 38and thence to a tube 40 connected to a vacuum pump or other suitablesource of vacuum. Oil vapor flowing to the trap 38 assists in drivingout the moisture in the tank. A vacuum may be applied through the vacuumpipe 36 previous to the spraying of hot oil over the coil and core orsimultaneously therewith or, if desired, application of vacuum may bedelayed until the coil and core have reached a substantial temperature.The most rapid drying out of transformers has been secured when theevacuation is begun at or before the time that hot oil is sprayed on thecoil and core. There are substantial quantities of moisture in theinsulation which require prolonged vacuum treatment to be applied toremove it to the necessary degree and a substantial period of time isrequired to accomplish this.

It is necessary that the introduction of the hot oil or other dielectricliquid through the conduit 20 and withdrawal of the cooled collecteddielectric liquid by the conduit 28 be so correlated that there will beno substantial portion of the coil and core immersed in the dielectricliquid during the process. Any substantial amount of immersion of thecoil and core will develop a hydrostatic pressure in that portion of theapparatus which is immersed and will hinder the withdrawal of moistureand gases therefrom.

The following is one example of the practice of the present process. Atransformer comprising a sealed casing was connected to a vacuum pumpand the internal pressure in the casing was reduced in 8 hours tobetween to 10 millimeters of mercury absolute pressure. Oil heated to100 C. was sprayed over the coil and core in the casing, and the coiland core were heated to a temperature of approximately 85 C. in another12 hours and the coil and core maintained at this temperature for 12hours, vacuum being maintained throughout this time. The spraying of oilwas then discontinued and all of the oil within the casing waswithdrawn. The transformer was then filled with refined transformer oilto the normal operating level. Tests indicated that the transformer wascomparable in electrical properties to one that had been dried byconventional processes which took approximately three times as long tocarry out.

The same process can be applied with considerable advantages toelectrical apparatus that has been in service and has been repaired orreconditioned. The oil adhering to the apparatus does not constitute anuisance or detriment to the drying out process of this invention. Inparticular the drying out process disclosed herein will not causeoxidation of the oil.

It should be understood that the above description and drawing areillustrative of the invention and not in limitation thereof.

I claim as my invention:

1. In the process of drying out electrical apparatus disposed within asealed casing, the steps comprising introducing into the sealed casing adielectric liquid heated to a temperature of at least C. but notexceeding C., the dielectric liquid having a low vapor pressure at thesetemperatures, distributing streams of the hot dielectric liquid over theelectrical apparatus whereby the apparatus is heated by the distributeddielectric liquid, the hot dielectric liquid picking up moisture andother impurities from the electrical apparatus in passing over it, thecooled dielectric liquid running off the electrical apparatus andcollecting in the bottom of the casing, withdrawing the cooleddielectric liquid from the bottom of the casing, the rate ofintroduction and withdrawal of the dielectric liquid being such that atany time there is insufficient dielectric liquid present in the casingto immerse any substantial portion of the electrical apparatus, passingthe withdrawn dielectric liquid through a purifying means and a heatingmeans to remove any impurities present in the dielectric liquid and toreheat the dielectric liquid to a temperature of at least 80 C. forreintroduction into the sealed casing, and continuing the introductionand withdrawal of the heated liquid into the casing, and evacuating thesealed casing to remove moisture, gases and other volatile impurities,the dielectric liquid not boiling under the temperatures and pressurespresent within the casing.

2. In the process of drying out electrical apparatus disposed within asealed casing, the steps comprising introducing within the sealed casingoil heated to a temperature of at least 80 C. but not exceeding 110 C.,the oil having a low vapor pressure at these temperatures, spraying thehot oil over the electrical apparatus within the casing, the amount ofoil within the casing at any one time being insuflicient to immerse anyportion of the electrical apparatus, the sprayed oil imparting heat tothe electrical apparatus as it flows over it and runs off into thebottom of the casing, the hot oil picking up moisture and otherimpurities for the electrical apparatus in passing over it, removing thecooled oil in the bottom of the casing and passing it to a purifyingmeans and a heating means to remove any impurities that may be presentin the oil and to reheat the oil to at least 80 C. for return to thesealed casing, respraying the reheated oil and continuing the cycle, andevacuating the casing during the process to remove moisture, gases andother volatile impurities therefrom.

3. In the process of drying out electrical apparatus disposed in asealed casing, the steps comprising distributing over the electricalapparatus in the casing a stream of a dielectric liquid at a temperatureof above 50 C. but not exceeding 110 C., the dielectric liquid having alow vapor pressure at these temperatures, whereby heat is imparted bythe heated dielectric liquid to the electrical apparatus, the hot oilpicking up moisture and other impurities from the electrical apparatusin passing over it, collecting the cooled dielectric liquid in thebottom of the casing after it has run off the apparatus, the quantity ofdielectric liquid present in the casing at any one time beinginsufiicient to immerse any substantial part of the electricalapparatus, passing the collected liquid through a purifying means and aheating means to remove any impurities that may have been picked up bythe dielectric liquid and to restore it to a temperature of above 50 C.,redistributing the reheated purified liquid over the electricalapparatus and continuing the steps of collecting, purifying, heating anddistributing the dielectric liquid, and maintaining a vacuum within thecasing during the process to withdraw any moisture, gases and impuritiesthat volatilize under the temperature and vacuum present in the casing.

4. In the process of drying out a transformer comprising a coil and acore disposed within a sealed casing prior to putting the transformer inservice, the steps comprising heating oil to a temperature of from 80 C.to 110 C., the oil having a low vapor pressure at these temperatures,

spraying the hot oil within the casing upon the coil and core wherebythe coil and core are heated rapidly thereby, no substantial part of theelectrical apparatus being immersed in the oil, evacuating the sealedcasing to an absolute pressure of not over 5 inches of mercury to removemoisture and other volatiles from the casing, the sprayed oil running01f the coil and core and collecting in the bottom of the casing, thehot oil picking up moisture and other impurities from the coil and corein passing over them, withdrawing the collected oil from the casing,passing the withdrawn oil through a body of absorbent material to purifythe oil, reheating the oil to a temperature ofi from 80 C. to 110 C.,respraying the hot oil Within the casing, and continuing this cycleuntil the transformer is dried out.

References Cited in the file of this patent UNITED STATES PATENTS1,835,922 Zeiss et al. Dec. 8, 1931 2,086,072 Fauth et al. July 6, 19372,293,453 Clark Aug. 18, 1942 2,500,783 Anderson et al Mar. 14, 1950

